Epitaxial structure

ABSTRACT

An epitaxial structure includes a substrate, a nucleation layer on the substrate, a buffer layer on the nucleation layer, and a nitride layer on the buffer layer. The nucleation layer consists of regions in a thickness direction, wherein a chemical composition of the regions is Al (1−X) In X N, where 0≤x≤1. A maximum value of the x value in the regions decreases along the thickness direction, and the x value in the chemical composition of each two regions consists of a fixed region and a gradient region, wherein a gradient slope of the gradient regions is −0.1%/nm to −50%/nm, and a stepwise slope of the fixed regions is −0.1%/loop to −50%/loop. A thickness of the nucleation layer is less than that of the buffer layer. A surface roughness of the nucleation layer in contact with the buffer layer is greater than that of the buffer layer in contact with the nitride layer.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a divisional application of and claims the prioritybenefit of U.S. patent application Ser. No. 16/452,558, filed on June26, 2019, now allowed. The prior application serial no. 16/452,558claims the priority benefit of Taiwan application serial no. 107126691,filed on Aug. 1, 2018. The entirety of each of the above-mentionedpatent applications is hereby incorporated by reference herein and madea part of this specification.

BACKGROUND Technical Field

The disclosure relates to a semiconductor structure, and moreparticularly, to an epitaxial structure.

Description of Related Art

Since the film formed by the epitaxy process has advantages of highpurity, good thickness control, etc., it has been widely applied to theproduction of radio-frequency (RF) components or power components.

In the epitaxial structure adopted in the general RF component, analuminum nitride (AlN) layer as a nucleation layer is formed on theSi-substrate before the epitaxial process is performed. However,spontaneous polarization induced by the nucleation layer material itselfoften occurs at the interface between the nucleation layer and theSi-substrate, or lattice mismatch between the nucleation layer and thesubstrate may induce piezoelectric polarization, which results in thepresence of a parasitic channel. As a result, the RF loss is increased.In addition, the aluminum atoms in the nucleation layer are prone todiffusion to the surface of the Si-substrate, which results in formationof a highly conductive layer, generates leakage current, and affects theRF component characteristics.

SUMMARY

The disclosure provides an epitaxial structure which can solve thediffusion of metal atoms in a nucleation layer to a substrate as in aconventional epitaxial structure, reduce the RF loss, and thus does notaffect the RF component characteristics.

An epitaxial structure of the disclosure includes a substrate, anucleation layer, a buffer layer, and a nitride layer. The nucleationlayer is disposed on the substrate. The nucleation layer consists of aplurality of regions in a thickness direction, and a chemicalcomposition of the regions is Al_((1−x))In_(X)N, where 0≤x≤1. A maximumvalue of the x value in the plurality of regions decreases along thethickness direction, and the x value in the chemical composition of eachtwo of the regions consists of a fixed region and a gradient region,wherein a gradient slope of the gradient region is −0.1%/nm to −50%/nm,and a stepwise slope of the fixed regions in the plurality of regions is−0.1%/loop to −50%/loop. The buffer layer is disposed on the nucleationlayer, and the nitride layer is disposed on the buffer layer. Athickness of the nucleation layer is less than a thickness of the bufferlayer. A roughness of a surface of the nucleation layer in contact withthe buffer layer is greater than a roughness of a surface of the bufferlayer in contact with the nitride layer.

In an embodiment of the disclosure, the gradient slope of the gradientregion is −0.5%/nm to −10%/nm, and the stepwise slope of the fixedregions in the plurality of regions is −0.1%/loop to −20%/loop.

In an embodiment of the disclosure, an initial content of the x value ofthe nucleation layer is 10% to 100%, an end content of the x value is 0%to 90%, and an initial content of the (1−x) value is 0% to 90%, and anend content of the (1−x) value is 10% to 100%, wherein the initialcontent of the x value is located on a bottom portion of the nucleationlayer close to the substrate, and the end content of the x value islocated on a top portion of the nucleation layer close to the bufferlayer.

In an embodiment of the disclosure, the initial content of the x valueof the nucleation layer is 50% to 100%, the end content of the x valueis 0% to 50%, the initial content of the (1−x) value is 0% to 50%, andthe end content of the (1−x) value is 50% to 100%.

In an embodiment of the disclosure, the thickness of the nucleationlayer is 1 nm to 500 nm.

In an embodiment of the disclosure, the thickness of the nucleationlayer is 1 nm to 50 nm.

In an embodiment of the disclosure, a number of the plurality of regionsof the nucleation layer is 2 to 100.

In an embodiment of the disclosure, a number of the plurality of regionsof the nucleation layer is 2 to 20.

In an embodiment of the disclosure, the roughness of the surface of thenucleation layer in contact with the buffer layer is 1 nm to 10 nm.

In an embodiment of the disclosure, the roughness of the surface of thenucleation layer in contact with the buffer layer is 1 nm to 3 nm.

Based on the above, in the epitaxial structure of the disclosure, thenucleation layer includes a plurality of regions, the chemicalcomposition of each of the regions is Al_((1−x))In_(X)N, and the x valuehas different types of variation along the thickness direction.Therefore, it is possible to solve the issue of the presence of theparasitic channel resulting from large spontaneous polarization as wellas piezoelectric polarization generated by lattice mismatch in thenucleation layer and the Si-substrate in the conventional epitaxialstructure. Moreover, it is also possible to solve the issue of reducedinterface resistance caused by the diffusion of atoms of the nucleationlayer to the Si-substrate, which further reduces the RF loss withoutaffecting the RF component characteristics.

To make the aforementioned more comprehensible, several embodimentsaccompanied with drawings are described in detail as follows.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings are included to provide a furtherunderstanding of the disclosure, and are incorporated in and constitutea part of this specification. The drawings illustrate exemplaryembodiments of the disclosure and, together with the description, serveto explain the principles of the disclosure.

FIG. 1 is a schematic cross-sectional view of an epitaxial structureaccording to an embodiment of the disclosure.

FIG. 2 is a schematic view showing a nucleation layer in a fixed contentvariation in an epitaxial structure according to the above embodiment.

FIG. 3 is a schematic view showing a nucleation layer in a gradientcontent variation in an epitaxial structure according to the aboveembodiment.

FIG. 4 is a schematic view showing a nucleation layer in a stepwisecontent variation in an epitaxial structure according to the aboveembodiment.

FIG. 5 is a schematic view showing a nucleation layer in a stepwisegradient content variation in an epitaxial structure according to theabove embodiment.

FIG. 6 is a schematic view showing a nucleation layer in a periodiccontent variation in an epitaxial structure according to the aboveembodiment.

FIG. 7 is a schematic view showing a nucleation layer in a periodicgradient content variation in an epitaxial structure according to theabove embodiment.

FIG. 8 is a schematic view showing a nucleation layer in a fullygradient content variation in an epitaxial structure according to theabove embodiment.

FIG. 9 is a schematic view showing a nucleation layer in another fullygradient content variation in an epitaxial structure according to theabove embodiment.

FIG. 10 is a schematic view showing a nucleation layer in still anotherfully gradient content variation in an epitaxial structure according tothe above embodiment.

DESCRIPTION OF THE EMBODIMENTS

Some embodiments are provided hereinafter and described in detail withreference to the accompanying drawings. However, the embodimentsprovided are not intended to limit the scope of the disclosure.Moreover, the drawings are only descriptive and are not drawn to scale.To facilitate understanding, the same components will hereinafter bedenoted by the same reference numerals.

FIG. 1 is a schematic cross-sectional view of an epitaxial structureaccording to an embodiment of the disclosure.

Referring to FIG. 1, the epitaxial structure of the present embodimentincludes a substrate 100, a nucleation layer 102, a buffer layer 104, anitride layer 106, and a barrier layer 108. The material of thesubstrate 100 is, for example, Si, Al₂O₃, SiC, GaAs, or other suitablematerial.

The nucleation layer 102 is disposed on the substrate 100. The thicknessof the nucleation layer 102 is less than the thickness of the bufferlayer 104, and the roughness of a surface 102 a of the nucleation layer102 in contact with the buffer layer 104 is greater than the roughnessof a surface 104a of the buffer layer 104 in contact with the nitridelayer 106. The nucleation layer 102 consists of a plurality of regions110 in the thickness direction. The chemical composition of theplurality of regions 110 is Al_((1−X))In_(X)N, where 0≤x≤1. The x valuerepresents the In (indium) content, and the (1−x) value represents theAl (aluminum) content. In addition, in the description herein, a“region” is defined as a variation in the x value, but the number of theregions does not necessarily represent the number of layers. Due to themanufacturing process, one single layer structure may contain multiplevariations in the x value. Therefore, one single layer may be composedof one single or a plurality of regions 110.

Although four regions 110 are shown in FIG. 1, the disclosure is notlimited thereto. In other embodiments, the number of the regions 110 is,for example, 2 to 100, or preferably, 2 to 20. In this range, moredesirable interface quality and two-dimensional electron gas (2DEG)characteristics can be obtained. The roughness (rms) of the surface 102a of the nucleation layer 102 in contact with the buffer layer 104 isgenerally between 1 nm and 10 nm, or preferably, between 1 nm and 3 nm.In this range, more desirable interface quality and 2DEG characteristicscan be obtained. The thickness of the nucleation layer 102 is, forexample, 1 nm to 500 nm, or preferably, 1 nm to 200 nm. In this range,more desirable interface quality and 2DEG characteristics can beobtained. In the present embodiment, by disposing the nucleation layer102, it is possible to reduce the amount of piezoelectric andspontaneous polarization, improve the highly conductive layer caused bythe parasitic channel, reduce the stress of the epitaxial structure,adjust the warpage of the epitaxial structure after epitaxial growth,and improve the crack length.

The buffer layer 104 is disposed on the nucleation layer 102. Thematerial of the buffer layer 104 is, for example, aluminum nitride(AlN). The roughness (rms) of the surface 104 a of the buffer layer 104in contact with the nitride layer 106 is, for example, between 0.2 nmand 3 nm and is less than the roughness of the surface 102 a of thenucleation layer 102, which will contribute to subsequent epitaxialgrowth of the nitride layer 106. The thickness of the buffer layer 104is generally greater than 500 nm. In the present embodiment, the bufferlayer 104 is disposed in the epitaxial structure as a stresscompensation adjustment and can adjust the warpage of the epitaxialstructure after epitaxial growth to thereby improve the crack length.The nitride layer 106 is disposed on the buffer layer 104. The materialof the nitride layer 106 is, for example, gallium nitride (GaN),aluminum nitride (AlN), indium nitride (InN), or aluminum gallium indiumnitride (AlGaInN). In addition, in an embodiment, the epitaxialstructure may also be provided with the barrier layer 108 on the nitridelayer 106. The material of the barrier layer 108 is, for example,aluminum nitride (AlN), indium nitride (InN), aluminum gallium indiumnitride (AlGaInN), or aluminum indium nitride (AlInN).

FIG. 2 to FIG. 10 are schematic views showing various content variationsof a nucleation layer in an epitaxial structure of the above embodiment,where “content” refers to the x value.

FIG. 2 is a schematic view showing a nucleation layer in a fixed contentvariation. Referring to FIG. 2, the fixed content variation is definedas a variation in which the x value in the chemical compositionAl_((1−x))In_(x)N of the plurality of regions 110 remains unchangedalong the thickness direction of the nucleation layer, and the x valueis not smaller than 10% (i.e., the indium content ratio is not smallerthan 10%), or preferably not smaller than 60%. In this range, moredesirable interface quality and 2DEG characteristics can be obtained.The thickness of the nucleation layer is, for example, 1 nm to 500 nm,or preferably 1 nm to 200 nm. The surface roughness of the nucleationlayer is, for example, 1 nm to 10 nm, or preferably 1 nm to 3 nm. Theprocess temperature of the nucleation layer is generally 500° C. to 850°C., or preferably 500° C. to 700° C. In this range, more desirableinterface quality and 2DEG characteristics can be obtained.

FIG. 3 is a schematic view showing a nucleation layer in a gradientcontent variation.

Referring to FIG. 3, the gradient content variation is defined as avariation in which the x value in the chemical compositionAl_((1−X))In_(x)N in the plurality of regions 110 decreases linearlyalong the thickness direction, and the gradient slope is, for example,−0.1%/nm to −50%/nm, or preferably −0.5%/nm to −10%/nm. In this range,more desirable interface quality and 2DEG characteristics can beobtained. Moreover, the initial content of the x value is, for example,10% to 100%, or preferably 50% to 100%. In this range, more desirableinterface quality and 2DEG characteristics can be obtained. The endcontent of the x value is, for example, 0% to 90%, or preferably 0% to50%. In this range, more desirable interface quality and 2DEGcharacteristics can be obtained. The initial content of the (1−x) valueis, for example, 0% to 90%, or preferably 0% to 50%, and the end contentof the (1−x) value is, for example, 10% to 100%, or preferably 50% to100%. The thickness of the nucleation layer is, for example, 1 nm to 500nm, or preferably 1 nm to 200 nm, and the surface roughness of thenucleation layer is, for example, 1 nm to 10 nm, or preferably 1 nm to 3nm.

In another embodiment, the x value has an initial content of 100% and anend content of 0%. The (1−x) value has an initial content of 0% and anend content of 100%.

FIG. 4 is a schematic view showing a nucleation layer in a stepwisecontent variation.

Referring to FIG. 4, the stepwise content variation is defined as avariation in which the x value in the chemical compositionAl_((1−x))In_(X)N in the plurality of regions 110 decreases stepwisealong the thickness direction, the x value does not change along thethickness direction in each of the regions 110, and the stepwise slopeof the plurality of regions 110 is −0.1%/loop to −50%/loop, orpreferably −0.1%/loop to −20%/loop. In this range, more desirableinterface quality and 2DEG characteristics can be obtained. Herein, theterm “loop” means that there are two different contents (i.e., high andlow contents) that are stacked periodically. Moreover, the initialcontent of the x value is, for example, 10% to 100%, or preferably 50%to 100%, and the end content of the x value is, for example, 0% to 90%,or preferably 0% to 50%. The initial content of the (1−x) value is, forexample, 0% to 90%, or preferably 0% to 50%, and the end content of the(1−x) value is, for example, 10% to 100%, or preferably 50% to 100%. Thethickness of the nucleation layer is, for example, 1 nm to 500 nm, orpreferably 1 nm to 50 nm. The number of the regions 110 in thenucleation layer is 2 to 100, or preferably 2 to 20. In this range, moredesirable interface quality and 2DEG characteristics can be obtained.The surface roughness of the nucleation layer is, for example, 1 nm to10 nm, or preferably 1 nm to 3 nm.

FIG. 5 is a schematic view showing a nucleation layer in a stepwisegradient content variation. Referring to FIG. 5, the stepwise gradientcontent variation is defined as a variation in which the maximum valueof the x value in the chemical composition Al_((1−x))In_(x)N in theplurality of regions 110 decreases along the thickness direction, andthe x value in the chemical composition of each two regions 110 consistsof a fixed region and a gradient region. In other words, each tworegions 110 are composed of a fixed region having the x value as a fixedvalue and a gradient region having the x value decreasing linearly. InFIG. 5, the x value of the fixed regions decreases stepwise, and thestepwise slope of the fixed regions is, for example, −0.1%/loop to−50%/loop, or preferably −0.1%/loop to −20%/loop. The gradient slope ofthe gradient regions is, for example, −0.1%/nm to −50%/nm, or preferably−0.5%/nm to −10%/nm.

In the stepwise gradient variation, the initial content of the x valueis, for example, 10% to 100%, or preferably 50% to 100%, and the endcontent of the x value is, for example, 0% to 90%, or preferably 0% to50%. The initial content of the (1−x) value is, for example, 0% to 90%,or preferably 0% to 50%, and the end content of the (1−x) value is, forexample, 10% to 100%, or preferably 50% to 100%. The thickness of thenucleation layer is, for example, 1 nm to 500 nm, or preferably 1 nm to50 nm, and the number of the regions 110 of the nucleation layer is, forexample, 2 to 100, or preferably 2 to 20. The surface roughness of thenucleation layer is, for example, 1 nm to 10 nm, or preferably 1 nm to 3nm.

FIG. 6 is a schematic view showing a nucleation layer in a periodiccontent variation. Referring to FIG. 6, the x values in the chemicalcomposition of each two regions 110 are respectively two fixed valuesand are stacked along the thickness direction. The initial content ofthe x value is, for example, 10% to 100%, or preferably 50% to 100%, andthe end content of the x value is, for example, 0% to 90%, or preferably0% to 50%. Herein, the term “initial content” refers to the contact endbetween the nucleation layer and the substrate as the initial position,and “end content” refers to the contact end between the nucleation layerand the buffer layer as the end position. The initial content of the(131 x) value is, for example, 0% to 90%, or preferably 0% to 50%, andthe end content of the (1−x) value is, for example, 10% to 100%, orpreferably 50% to 100%. The thickness of the nucleation layer is, forexample, 1 nm to 500 nm, or preferably 1 nm to 50 nm, and the number ofthe regions 110 of the nucleation layer is, for example, 2 to 100, orpreferably 2 to 20. The surface roughness of the nucleation layer is,for example, 1 nm to 10 nm, or preferably 1 nm to 3 nm.

FIG. 7 is a schematic view showing a nucleation layer in a periodicgradient content variation. Referring to FIG. 7, the periodic gradientcontent variation is defined as a variation in which the x valueincreases or decreases periodically and gradually along the thicknessdirection. For example, the x value in the chemical compositionAl_((1−X))In_(X)N of each four regions 110 in the nucleation layerconsists of four sections of variation along the thickness direction,and the four sections of variation include: a first fixed region havinga fixed value which is the maximum value, a first gradient regiongradually changing from the maximum value to the maximum value, a secondfixed region having another fixed value which is the minimum value, anda second gradient region gradually changing from the minimum value tothe maximum value. The absolute value of the gradient slope of the firstgradient region and the second gradient region is, for example, 0.1%/nmto 50%/nm, or preferably 0.5%/nm to 10%/nm.

In the periodic gradient variation, the initial content of the x valueis, for example, 10% to 100%, or preferably 50% to 100%, and the endcontent of the x value is, for example, 0% to 90%, or preferably 0% to50%. The initial content of the (1-x) value is, for example, 0% to 90%,or preferably 0% to 50%, and the end content of the (1-x) value is, forexample, 10% to 100%, or preferably 50% to 100%. The thickness of thenucleation layer is, for example, 1 nm to 500 nm, or preferably 1 nm to50 nm, and the number of the regions 110 of the nucleation layer is, forexample, 4 to 100, or preferably 4 to 20. The surface roughness of thenucleation layer is, for example, 1 nm to 10 nm, or preferably 1 nm to 3nm.

FIG. 8, FIG. 9, and FIG. 10 are schematic views showing a nucleationlayer in three fully gradient content variations.

The fully gradient content variation is defined as a variation in whichthe x value in the chemical composition of each of the regions 110increases or decreases fully gradually along the thickness direction.

Referring to FIG. 8, the maximum value of the x value in the regions 110is the same, and the minimum value of the x value is the same. Moreover,the absolute value of the gradient slope of each of the regions 110 is,for example, 0.1%/nm to 50%/nm, or preferably 0.5%/nm to 10%/nm. Theinitial content of the x value is, for example, 10% to 100%, orpreferably 50% to 100%, and the end content of the x value is, forexample, 0% to 90%, or preferably 0% to 50%. The initial content of the(1-x) value is, for example, 0% to 90%, or preferably 0% to 50%, and theend content of the (1-x) value is, for example, 10% to 100%, orpreferably 50% to 100%.

The thickness of the nucleation layer is, for example, 1 nm to 500 nm,or preferably 1 nm to 50 nm. The number of the regions 110 of thenucleation layer is, for example, 2 to 100, or preferably 2 to 20. Thesurface roughness of the nucleation layer is, for example, 1 nm to 10nm, or preferably 1 nm to 3 nm.

In FIG. 9, the maximum value of the x value in the plurality of regions110 decreases along the thickness direction, and the minimum value ofthe x value is the same. The absolute value of the gradient slope ofeach of the regions 110 is, for example, 0.1%/nm to 50%/nm, orpreferably 0.5%/nm to 10%/nm, and the absolute value of the stepwiseslope of the regions 110 is, for example, 0.1%/loop to 50%/loop, orpreferably 0.5%/loop to 10%/loop. The initial content of the x value is,for example, 10% to 100%, or preferably 50% to 100%, and the end contentof the x value is, for example, 0% to 90%, or preferably 0% to 50%. Theinitial content of the (1-x) value is, for example, 0% to 90%, orpreferably 0% to 50%, and the end content of the (1-x) value is, forexample, 10% to 100%, or preferably 50% to 100%. The thickness of thenucleation layer is, for example, 1 nm to 500 nm, or preferably 1 nm to50 nm, and the number of the regions 110 of the nucleation layer is, forexample, 2 to 100, or preferably 2 to 20. The surface roughness of thenucleation layer is, for example, 1 nm to 10 nm, or preferably 1 nm to 3nm.

In FIG. 10, the maximum value of the x value in a plurality of regions110 a to 110 d decreases along the thickness direction, the minimumvalue of the x value is the same, and the x value in the chemicalcomposition of the four regions 110 a to 110 d consists of four sectionsof variation. The four sections of variation include: the first gradientregion 110 a gradually changing from the maximum value to the minimumvalue, the second gradient region 110 b gradually changing from theminimum value to the maximum value, the third gradient region 110 cgradually changing from the maximum value to the minimum value, and thefourth gradient region 110 d gradually changing from the minimum valueto the maximum value. The maximum value of the x value is the same inthe first gradient region 110 a, the second gradient region 110 b, andthe third gradient region 110 c. The maximum value of the x value in thefourth gradient region 110 d is a value that decreases from the maximumvalue of the x value of the first gradient region 110 a (the secondgradient region 110 b and the third gradient region 110 c) at a stepwiseslope of −0.1%/loop to −50%/loop, and the stepwise slope is preferably−0.5%/loop to −10%/loop. The absolute value of the gradient slope ofeach of the regions 110 a to 110 d is, for example, 0.1%/nm to 50%/nm,or preferably 0.5%/nm to 10%/nm.

In FIG. 10, the initial content of the x value is, for example, 10% to100%, or preferably 50% to 100%, and the end content of the x value is,for example, 0% to 90%, or preferably 0% to 50%. The initial content ofthe (1-x) value is, for example, 0% to 90%, or preferably 0% to 50%, andthe end content of the (1-x) value is, for example, 10% to 100%, orpreferably 50% to 100%. The thickness of the nucleation layer is, forexample, 1 nm to 500 nm, or preferably 1 nm to 50 nm. The number of theregions 110 a to 110 d of the nucleation layer is, for example, 4 to100, or preferably 4 to 20. In this range, more desirable interfacequality and 2DEG characteristics can be obtained. The surface roughnessof the nucleation layer is, for example, 1 nm to 10 nm, or preferably 1nm to 3 nm.

From the perspective of solving the RF loss, the embodiments of FIG. 8to FIG. 10 are preferred. The reason is that since the interface of thenucleation layer and the buffer layer is a continuous variation, thedefect density of the interface can be reduced, and the quality of theepitaxial material and the interface smoothness can be improved. Inaddition, since the interface is a continuous variation, the interfacestress generated is small, and the amount of polarization is relativelylow, which can effectively improve the highly conductive layer caused bythe parasitic channel and thereby reduce the RF loss.

In summary of the above, according to the epitaxial structure of thedisclosure, the issue of the conventional nucleation layer (AlN) can beimproved through the different content (x value) variations in thenucleation layer of Al_((1−x))In_(x)N. Table 1 below shows the expectedRF characteristics of the conventional nucleation layer and thenucleation layer of the disclosure applied to the epitaxial structure.

TABLE 1 Conventional Nucleation layer nucleation layer of the disclosureAlN Al_((1−x))In_(x)N Lattice matching Inferior Superior Spontaneouspolarization High Low Charge induced by spontaneous Much Littlepolarization Piezoelectric polarization High Low Charge induced bypiezoelectric Much Little polarization Total amount of polarization HighLow Al diffusion coefficient in High Low Si-substrate

According to Table 1, since the nucleation layer of the disclosure hasbetter lattice matching with the Si-substrate, lower spontaneouspolarization, and lower piezoelectric polarization, and can reduce thediffusion of aluminum in the Si-substrate, it is possible to reduce theRF loss and ensure the RF characteristics.

It will be apparent to those skilled in the art that variousmodifications and variations can be made to the disclosed embodimentswithout departing from the scope or spirit of the disclosure. In view ofthe foregoing, it is intended that the disclosure covers modificationsand variations provided that they fall within the scope of the followingclaims and their equivalents.

What is claimed is:
 1. An epitaxial structure comprising: a substrate; anucleation layer disposed on the substrate, wherein the nucleation layerconsists of a plurality of regions in a thickness direction, and achemical composition of the plurality of regions is Al_((1−X))In_(X)N,where 0≤x≤1, wherein a maximum value of the x value in the plurality ofregions decreases along the thickness direction, and the x value in thechemical composition of each two of the regions consists of a fixedregion and a gradient region, wherein a gradient slope of the gradientregion is −0.1%/nm to −50%/nm, and a stepwise slope of the fixed regionsin the plurality of regions is −0.1%/loop to −50%/loop; a buffer layerdisposed on the nucleation layer, wherein a thickness of the nucleationlayer is less than a thickness of the buffer layer; and a nitride layerdisposed on the buffer layer, wherein a roughness of a surface of thenucleation layer in contact with the buffer layer is greater than aroughness of a surface of the buffer layer in contact with the nitridelayer.
 2. The epitaxial structure according to claim 1, wherein thegradient slope of the gradient region is −0.5%/nm to −10%/nm, and thestepwise slope of the fixed regions in the plurality of regions is−0.1%/loop to −20%/loop.
 3. The epitaxial structure according to claim1, wherein an initial content of the x value of the nucleation layer is10% to 100%, an end content of the x value is 0% to 90%, and an initialcontent of the (1−x) value is 0% to 90%, and an end content of the (1−x)value is 10% to 100%, wherein the initial content of the x value islocated on a bottom portion of the nucleation layer close to thesubstrate, and the end content of the x value is located on a topportion of the nucleation layer close to the buffer layer.
 4. Theepitaxial structure according to claim 3, wherein the initial content ofthe x value of the nucleation layer is 50% to 100%, the end content ofthe x value is 0% to 50%, the initial content of the (1−x) value is 0%to 50%, and the end content of the (1−x) value is 50% to 100%.
 5. Theepitaxial structure according to claim 1, wherein the thickness of thenucleation layer is 1 nm to 500 nm.
 6. The epitaxial structure accordingto claim 5, wherein the thickness of the nucleation layer is 1 nm to 50nm.
 7. The epitaxial structure according to claim 1, wherein a number ofthe plurality of regions of the nucleation layer is 2 to
 100. 8. Theepitaxial structure according to claim 7, wherein the number of theplurality of regions of the nucleation layer is 2 to
 20. 9. Theepitaxial structure according to claim 1, wherein the roughness of thesurface of the nucleation layer in contact with the buffer layer is 1 nmto 10 nm.
 10. The epitaxial structure according to claim 9, wherein theroughness of the surface of the nucleation layer in contact with thebuffer layer is 1 nm to 3 nm.